Rapid developments in digital signal processing is one of the main development drivers in many systems containing acoustic detectors. In these types of systems 100 (see FIG. 1), an analog signal from an acoustic transducer 101 is normally converted into the digital domain using some analog signal conditioning circuit 102 and a multi stage delta-sigma converter 103. The digital signal is then processed by a processor 104 and subsequently converted back to an analog signal 105 for the loudspeaker 106 in the system 100.
Delta-sigma signal conversion has received much attention as the operating frequency of integrated circuits continues to increase. The conversion method is important because it allows one to shift noise sources out of the frequency band of interest. An important detail of the system, however, is that an analog front-end circuit 102 is required to convert the change of microphone capacitance into an electrical signal with the lowest possible addition of electrical noise. In addition, if condenser microphones are used, a circuit 107 to provide a DC bias voltage is also required. In electret microphones, the bias is provided internally and therefore only an amplifier is required.
Capacitive transducers are used to detect a variety of physical conditions, such as temperature, pressure (sound), and force (acceleration). The use of capacitive transducers is very prevalent because they provide unparalleled performance in terms of size, low inherent self-noise and power consumption. The problem with capacitive transducers, however, is that the output impedance is so high that the signal conditioning circuit 102 used to read out the transducer signal must have extremely high input impedance, so that it does not load, and thereby dampen, the already small sensor signal.
Three typical signal detection circuits are shown in FIGS. 2–4. The circuit shown in FIG. 2 is a transconductance amplifier 200, which has an inherent gain of less than 1 (typically about 0.8). In other words, the amplifier 200 transforms the signal from transducer 202 to a low impedance output at the expense of some of the signal level. The advantages of this circuit, apart from its simplicity, are low noise, very low power consumption and very low operating voltage. The most important drawbacks are poor power supply rejection and sensitivity to parasitic capacitances (Cp in FIG. 2) between the transducer capacitance Ct and the amplifier 200. Any parasitic capacitance will load the sensor signal leading to a gain factor K:                     K        =                                            C              t                                                      C                t                            +                              C                p                                              <          1                                    (        1        )            
The circuit shown in FIG. 3, which is often referred to as a charge amplifier 300, overcomes the shortcomings of the transconductance amplifier 200 of FIG. 2. Since a differential amplifier 301 is used in circuit 300, the principle of virtual ground will exist on the input of the amplifier 301, which means the effect of the parasitic capacitance Cp will be reduced by the open loop gain of the differential amplifier 301. Most differential amplifiers have open loop gains in excess of 100 dB, which makes the parasitic capacitance practically irrelevant. Secondly, since the differential amplifier 301 is a more sophisticated device than the simple transconductance amplifier 200 of FIG. 2, the power supply rejection will be at least 40–50 dB better. The gain of charge amplifier 300 is determined by the ratio of the capacitance Ct of transducer 302 over the amplifier feedback capacitance Cfb, and can often be designed to be larger than 1. The feedback resistance Rfb is necessary to ensure DC stability, and must be chosen large enough to ensure that the cut-off frequency of the filter Rfb and Cfb is outside the bandwidth of interest in transducer 302. The most important drawbacks of the charge amplifier circuit 300 are the power consumption and the relatively high required supply voltage. In addition, the self-noise of the charge amplifier 300 is higher than the transconductance amplifier 200. Since noise is normally a very important parameter, the transconductance amplifier 200 is preferred for many applications.
A possible solution to the noise problem in charge amplifier 300 is shown in FIG. 4. In the circuit 400 of FIG. 4, a harmonic voltage source Vsine is used to bias the transducer 402. The harmonic source operates at a much higher frequency than the maximum frequency of interest in transducer 402. The signal from the harmonic source, called the carrier signal, experiences a gain in charge amplifier 400, which depends on the ratio of the transducer capacitance Ct over the feedback capacitance Cfb. As the transducer capacitance changes, the amplifier gain changes, and hence the amplitude of the output signal Vout changes. This constitutes a simple amplitude modulation (AM) circuit, and it is useful since it shifts the frequency of operation in charge amplifier 400 to a much higher frequency. Since the self-noise in charge amplifier 400 is dominated by 1/f noise, in effect, this circuit moves the frequency of operation to a regime where the self-noise is much lower, leading to improved performance. The amplitude modulated transducer signal can be demodulated with a simple low-pass filter. The major drawbacks of the AM circuit are high power consumption, high required supply voltage, and the need for a harmonic voltage source. Furthermore, the harmonic voltage source must have very stable amplitude, the variations of which will show as artifacts in the demodulated transducer signal. Implementing harmonic voltage sources with low amplitude noise is a challenging task.
With the incredible rise of digital circuit technology, microprocessors, and other very high speed digital processing circuits, the use of digital signal processing is now one of the most important technologies for instrumentation and communication applications. Two major digital signal processing applications in which capacitive transducers are also used are digital hearing aids and mobile phones. In both applications, an acoustic analog signal is converted to the digital domain, and then processed, filtered and transmitted. In FIG. 5, a commonly used circuit 500 is shown for conversion of the signal from the capacitive transducer 502 from the acoustic domain to the analog electrical domain with an amplifier 501, like those previously shown in FIGS. 2–4, and then to the digital domain using a delta-sigma converter 503 of the analog signal. The result is a high frequency digital bit stream 504, in which the transducer signal is quantized and represented as a sequential stream of bits suitable for a direct feed to a digital signal processor (“DSP”), such as that shown in FIG. 1. While circuit 500 can be built to perform with very low self-noise, it requires analog circuitry and amplifiers, which cannot be readily integrated with a DSP on the same chip, due to limitations in integrated circuit manufacturing. A possible solution to this problem is the circuit 600 shown in FIG. 6, in which no analog circuitry is used with the transducer 602. The capacitive transducer 602 is connected to a ring oscillator 601 which has a digital output with a frequency given by:                               f          osc                =                  1                                    R              osc                        ⁢                          C              t                        ⁢                          ln              ⁡                              [                                                                            (                                                                        2                          ⁢                                                      V                            sup                                                                          -                                                  V                          sw                                                                    )                                        ⁢                                          (                                                                        V                          sup                                                +                                                  V                          sw                                                                    )                                                                                                  (                                                                        V                          sup                                                -                                                  V                          sw                                                                    )                                        ⁢                                          V                      sw                                                                      ]                                                                        (        2        )            where Rosc is the oscillator timing resistance, Vsup is the supply voltage, and Vsw is the switching voltage of the inverters 603. As the capacitance Ct of transducer 602 changes, the oscillator frequency changes according to equation (2), and the result is an output with a frequency modulated (FM) representation of the transducer signal. The problem with this digital detection circuit 600 is that the FM output is not directly compatible with the input required on a DSP, and would hence require extra processing, such as up or down conversion, to be used.